Swept range gate radar system for detection of nearby objects

ABSTRACT

A cost-effective ultra-wideband radar system capable of locating nearby buried objects such as reinforcing steel rods, pipes, and other objects buried in concrete, soil, behind walls, or in the air. A sequence of ultra-wideband radar pulses e.g. at a plurality of frequencies in a range of about 2 MHz to about 10 GHz are emitted without a carrier and the system detects deflected pulse energy caused by the transmitted pulse whenever encountering a change in the medium i.e. an air to metal change or concrete to metal change. This reflected energy is detected and visually displayed. The range gate delay of the receiver is continuously varied, thus changing the distance from the unit to where the reflected energy would be potentially detected from the target. By continuously sweeping the &#34;depth&#34; of the scan, the operator need only move the unit in two dimensions across the surface to detect objects buried or hidden at varying depths interior to or behind the surface. The range gate system includes a multipoint background subtraction, corrected gain with distance, linear range gate time correction and a dielectric constant correction for a calibrated distance display.

This application is a continuation of application Ser. No. 08/582,290,filed Jan. 3, 1996 now abandoned, which is a continuation of Ser. No.08/300,279, filed Sep. 2, 1994, now U.S. Pat. No. 5,543,799 issued Aug.6, 1996 to Heger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radar systems, and more specifically to ashort range and inexpensive radar system for use in locating reinforcingsteel rods, pipes, and other nearby objects buried in concrete, soil, orbehind walls and other applications.

2. Description of the Prior Art

A requirement exists for a cost-effective system capable of locatingreinforcing steel rods (rebar), pipes, and other objects buried inconcrete, soil, behind walls, etc. Various devices and systems currentlyexist to locate these various objects, but all are either limited incapability or very costly.

Zircon Corporation, Campbell, Calif., currently produces an electronicstud sensor for locating wooden and metal wall studs behind sheetrockand can sense studs up to 1" to 11/2" deep beneath other material suchas sheetrock, plywood, etc.

Lawrence Livermore National Laboratory (LLNL) has developed anultra-wideband (UWB) radar technology applicable to produce astud-sensing type of apparatus. See U.S. patent application Ser. No.08/058,398 entitled "Impulse Radar Stud Finder", filed May 7, 1993,invented by Thomas Edward McEwan, incorporated by reference.

See also U.S. patent application Ser. No. 08/044,717 entitled"Ultra-Wideband Motion Sensor," now U.S. Pat. No. 5,361,070; and Ser.No. 08/044,754 entitled "A Differential Receiver for Ultra-WidebandSignals filed Sep. 1, 1992, both invented by Thomas Edward McEwan andalso incorporated by reference. Unlike the Zircon product which reliesupon sensing a change in density via capacitive loading, theabove-described LLNL technology as shown in present FIG. 1 emits asequence of ultra-wideband radar pulses without a carrier and detectsdeflected pulse energy caused by the transmitted pulse wavefrontencountering a change in medium, i.e., air-to-metal orconcrete-to-metal, hereafter referred to as the target. This reflectedenergy is detected and visually displayed.

The LLNL technology generates a fast pulse (typically 100 to 1000 pswide) from a 1 MHz oscillator 10 driving a step generator 12 which iscoupled to a transmit antenna 14. A separate receive antenna 16 iscoupled to a sampler circuit 18 which is gated on with a delayed versionof the transmit pulse by a fixed range delay generator 22 driving asecond step generator 20. The receiver (including sampler circuit 18feeding amplifier and background subtract circuit 24 coupled to display26) thus "looks" for reflected energy at a fixed time delay after thetransmit pulse has occurred. This generates a so-called "range gate",which is well known in the radar field.

This fixed range gate allows detection of objects at a fixed physicaldistance from the unit as dictated by a) the unit's range gate delay, b)the material(s) the pulse energy is passing through and c) the speed oflight.

In a fairly fixed and repeatable situation such as sensing wall studsbehind sheetrock, the range gate delay time can be fixed at a standarddistance because little variation occurs in home and other buildingconstruction techniques. However, if a broad variety of sensingapplications is required such as those described above, then it has beenfound that a fixed range gate delay unit will not suffice. The operatorcould have a manually variable range gate control, but this wouldrequire: a) physical scanning with the unit in both vertical andhorizontal planes across a surface, b) changing the range gate controlat each point desired to scan and c) doing a new background subtract(recalibration) at each of these range gate settings. This would be avery tedious and error-prone type of operation.

SUMMARY OF THE INVENTION

The present invention improves upon the prior art fixed range gate delayapproach for UWB radar by continuously varying the range gate delay,thus changing the distance from the unit to where reflected energy wouldbe potentially detected from a target. By continuously sweeping the"depth" of the scan, the operator need only move the unit in twodimensions across a surface to detect objects buried or hidden atvarying depths interior to or behind the surface, or otherwise spacedapart from the unit.

Several technical problems have been both discovered and solved by thepresent inventor to accomplish this. First, at any given range gatedelay, it has been found that a certain amount of random, reflectedenergy will be returned to the receive antenna. This "clutter" is dueprimarily to finite transmit and receive antenna structure and circuitisolation. To accommodate a cost-effective solution without resorting tomuch more costly microwave techniques, a certain amount of this clutteris tolerated. The "background subtract" circuit in prior art FIG. 1 doesa single one-time subtraction at calibration and removes or subtractsthese residual, unwanted return signals which, for a single, fixed rangegate delay, will be fairly constant for any given physical orientationsuch as scanning a sheetrock wall.

If, however, the range gate is continuously varied, it has been foundthat the "background clutter" also continuously varies in a randomfashion in synchronicity with the range gate sweep. To overcome this, inaccordance with the invention the range gate is stepped in smallincrements and at each step, a unique background removal value is sensedand stored in an initial calibration routine. Once accomplished in anyphysical sensing situation, such as scanning a concrete wall, the storedbackground values are then subsequently recalled at each range gate stepand subtracted from whatever the actual return signal is at thatparticular range setting, thus leaving for display processing only thechange in reflected energy from a "calibrated" situation, such as thedetection of embedded rebar in the concrete.

A second problem found with sweeping the range gate is that thereflected energy from a "target" decreases (assuming constant pulsetransmit energy) as the square of the distance, as is well known forradio wave propagation. Therefore, a gain that varies proportionally tothe square of the range gate delay is incorporated to remove the varyingdetection distance from the amplitude of the reflected energy. This isimportant in that the amount of reflected energy has significance asindicating the size and material of the reflecting target.

Thirdly, it has been found that it is desirable that the range gatedelay vary in direct linear proportion to the range gate sweep control.This allows a direct calibration "depth" indication of the target fromthe unit. A correction factor has also been determined to be required toobtain a calibrated depth readout. The dielectric constant (e_(R)) ofthe bulk material through which the pulse is passing affects thevelocity of propagation by the square root of e_(R). Thus, to obtain theactual depth or distance from the sensing unit's antennas to thereflecting target, a process using the following calculation is used:

    D=(0.5)(t.sub.D)(C)(e.sub.R).sup.1/2

where

D=distance from antenna(s) to the target,

t_(D) =total round trip delay time of pulse,

e_(R) =bulk dielectric constant of medium, and

C=speed of light

The factor of 0.5 is required due to the pulse being a "round trip" fromthe transmit antenna to the target and back to the receive antenna.

To summarize, a swept range gate system in accordance with the inventionmay include at least these structures and corresponding methods beyondthat required for a prior art fixed range gate unit: linear range gatetime; corrected gain with distance; and multi-point backgroundsubtraction.

In addition, dielectric constant correction is provided for a calibrateddistance display.

While the UWB system disclosed herein operates over a range of 2 MHz to10 to 15 GHz, having harmonics at each multiple of 2 MHz, this is notlimiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art fixed range gate radar system.

FIG. 2 shows a block diagram of a swept range gate ultra-wideband radarsystem in accordance with the present invention.

FIG. 3 shows waveforms illustrating the swept range gate system timingof the system of FIG. 2.

FIG. 4 shows a diagram of a display as used in accordance with thesystem of FIG. 2.

FIG. 5A, 5B and 5C show examples of a display corresponding toparticular detection situations in accordance with the presentinvention.

FIG. 6 shows a voltage controlled delay generator in accordance with thepresent invention.

FIG. 7 shows an antenna structure in accordance with the presentinvention.

FIG. 8 shows a transmitter in accordance with the present invention.

FIG. 9 shows a receiver in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The various features in accordance with the invention as described abovemay be accomplished by a combination of analog and/or digital circuitryand/or software executed e.g. in a microprocessor. FIG. 2 shows oneembodiment of a system 32 that includes these features.

A 2 MHz master oscillator 36 drives two paths: a first path via avoltage controlled variable delay circuit 38 and then to the transmitpulse generator 40, and a second path through a fixed delay circuit 42and then to the receive pulse generator 44. The fixed delay circuit 42is similar to variable delay circuit 38, without the variable elements(see below, FIG. 6).

The voltage controlled variable delay 38 allows changing the time of thetransmit pulse with respect to the receive pulse generator 44, with adigital-to-analog converter (D/A) 50 providing a varying voltage undercontrol of a conventional microcontroller 52. This structure thengenerates a continuously changing time delay from some minimum tomaximum delay time, creating the swept range. A typical sweep frequencyis 100 Hz.

The fixed delay in the receiver path is set equal to or greater than themaximum transmit delay. The timing relationship of the transmit pulsegenerator 40 and receive pulse generator 44 then determines the distancerange over which the unit will scan.

FIG. 3 depicts this timing by waveforms showing the output signals ofvarious portions of the unit 32. The configuration of FIGS. 2 and 3 isthus opposite that conventionally used in radar systems where thetransmit pulse time is fixed and the receiver gate time is varied. (Theconventional configuration may be utilized for the present invention butis not the preferred embodiment.) Sweeping the receive delayconventionally has been found to create additional background clutterdue to extraneous signal coupling in the presence of high gainamplifiers and "droop" on the sampler hold capacitor 56, undesirablycreating a phase-modulated replica of the range sweep control voltage.These disadvantages are eliminated when the transmit pulse is variedinstead.

With reference to FIG. 3, times t_(D1) and t_(D2) are the minimum andmaximum respectively of the transmit voltage-controlled pulse delay 38.The receive pulse delay 42 is fixed at t_(D3). Thus, the minimum rangeis set by t_(D3) -t_(D2) =t_(D4) (which could be equal to zero). Themaximum range is t_(D3) -t_(D1). The transmit delay can be any valuebetween these limits as defined by the microcontroller 52 via the D/A 50driving the voltage variable delay circuit 38.

In operation, when the delay between the transmit 40 and receive pulsegenerator 44 is equal to the round trip transit time of the transmitpulse reflecting off a target, the receive sampler 54 will be turned oncoincident with the arrival of the return echo and the sampled and heldsignal is amplified by amplifier 58.

Prior to any actual sensing, a calibration process is performed toremove the background "clutter". At each finite range gate controlvoltage from D/A 50, with the gain of voltage variable gain circuit 62set via D/A 64 to a previously determined setting, offset D/A 66 drivingthe offset voltage control of amplifier 58 is varied with a successiveapproximation routine (or other search routine resident inmicroprocessor 52) until the output of A/D 70 is at mid-range. Theoffset binary number (value) driving offset D/A 66 is then stored inmicrocontroller 52 memory and the range gate D/A 50 is stepped to thenext value along with the appropriate gain control D/A 64 binary number(value). The offset D/A 66 search routine is repeated and the numbersaved. This process is repeated until all range gate control settingshave been processed in this manner. (It is to be understood that writinga software routine carrying this out via microprocessor/microcontroller52 is well within the ability of one of ordinary skill in the art.) Thevariable gain circuit 62 is any circuit for varying a gain by a voltagecontrol. The unit 32 must be physically stable with relationship to asurface or pointed into the surface as this routine is performed. (Thiscalibration process is applicable, with appropriate readily apparentmodifications, to an UWB radar system where the receiver is range sweptrather than the transmitter.)

After initial calibration, normal operation can commence. At each rangegate D/A 50 setting, the offset D/A 66 and gain D/A 64 settingsassociated with this range are recalled by the microcontroller 52. Withno echo at any range setting, the output of A/D (analog to digitalconverter) 70 will always be at mid-scale, as any previous background issubtracted out by the offset D/A 66. If, however, an echo is presentrepresenting a target at some given distance and range gate setting, theA/D 70 output will not be mid-scale but some other value. Thisdifference from mid-scale is then processed by the microcontroller 52and displayed on graphics display 74.

Display

The described system provides a number from the A/D 70 related to thestrength of the return echo which is uniquely associated with a givenrange value. The display 74 (e.g., an LCD or other conventional display)therefore, has two pieces of information to visually display: amplitudeand range. All physical movement of the unit 32 over the surface beingscanned is done by the operator, with any given display presentationbeing uniquely associated with a given position of the unit on thesurface. Thus the operator has physical control of the X and Ycoordinates (i.e. the surface being scanned) and the unit 32 scanselectronically via the range gate the Z axis (i.e. into, behind, etc.,the surface being scanned). ("Surface" need not be a structure per sebut could be merely the surface of unit 32 where it is in contact withthe air.) Any change in amplitude on the display can now be associatedwith a unique point within the volume of the object being scanned (X, Y& Z) with amplitude giving an indication of size and material, ofobjects buried/hidden interior to the volume.

Since the operator is physically doing all X and Y axis scanning, thedisplay 74 should not reflect any bias to either X or Y movement toprevent any operator confusion or misinterpretation. FIG. 4 depicts aplanar graphics display as an example of display 74 of FIG. 2 and thathas two axes of information: range (Min to Max) and amplitude (zero toMax on either side of the centerline). The display 74, as physicallyattached to the unit, is orthogonal to the surface being scanned whenthe unit 32 is in use. The display 74 thus provides a representation ofthe cross-section of the volume being scanned at that physical locationon the surface. The display of amplitude information is in oneembodiment "mirror imaged" about the center line shown to eliminate anyX-Y bias. FIGS. 5A, 5B, 5C show several display examples.

FIG. 5A shows the display 74 of unit 32 when the volume being scannedcontains only homogeneous material within the scan range of the unit.

FIG. 5B shows the detection of metal rebar at depth D1 into the volumeand depicted on the display at position D1' indicating the depth.

FIG. 5C is similar to FIG. 5B but with the rebar at a greater depth D2,shown on display 74 at position D2'.

The number of range gate values scanned, the amplitude A/D 70resolution, number of display 74 pixels, range depth and other systemparameters can be varied to suit the intended application.

Range Gate Linearity

As previously mentioned, to obtain a calibrated range display, the rangegate (or voltage variable delay) must correlate to a given binary numbersent to the variable delay D/A 50. If the voltage-to-delay function ofthe voltage controlled delay 38 were linear, the D/A 50 output need onlybe scaled to obtain the required range. However, due to the very smalldelays required (typically less than 10 ns maximum which equates to 5feet round trip in air) a convenient method (see FIG. 6) to obtainvoltage controlled delay uses fast logic elements 80, 82 (gates such asbuffers, inverters, etc. of logic families such as HC and AC) andslightly varies the delay between gates 80, 82 with R-C(resistor-capacitor) delays as shown. Resistor R1 and capacitor C1 delaythe output of the first gate 80. Varying the control voltage on resistorR2 can slightly modify this delay. The voltage-to-delay transferfunction is not linear, however.

If the control voltage 50 were alternatively to be driven by a linearsawtooth analog voltage such as from an analog sawtooth generator, meansmust be provided (not shown) to linearize the time/voltage transferfunction.

However, in the system block diagram shown in FIG. 2, the binary word(value) sent to the delay control D/A 50 can be modified to other than alinear binary sequence to correct for any nonlinearities in the circuitof FIG. 6. This linearity correction is part of the microcontroller 50resident software and once determined for a given design of unit 32 forparticular type of logic elements, parts values, etc., would beidentical in all other units.

Dielectric Constant Correction

As stated above, the dielectric constant of bulk material being scannedaffects the pulse propagation time, and thus is taken into account inaccordance with the invention at reference number 86 of FIG. 2 torealize a calibrated "depth" display.

The dielectric constant of various materials is well known and anyconventional input device 86 to the microcontroller 52 such as rotaryswitches, a keypad, etc. (not shown) may be utilized to select theappropriate constant that the microcontroller 52 could then use in thedistance (depth) determination described above. A range of e_(R) from 1(air) to 80 (water) would cover most potential applications. A printedtable of various materials and their e_(R) 's is then attached to theexterior of unit 32, as well as possibly having user controlled switchpositions to select a variety of often used applications such asconcrete, 1/2" sheetrock, tile roof, etc. This would simplify the user'soperation of unit 32.

Antennas

The prior art device of FIG. 1 uses monopole antennas above a groundplane with no cavities or electro-magnetic field isolation from eachother.

It has been found by the present inventor that it is desirable toisolate the transmit and receive antennas as much as possible consistentwith other requirements (such as cost and size) so that the receiverneed not cope with the very large transmitted pulse, as compared to themuch weaker return echo. Also, to obtain the most unambiguous rangedetermination, the transmit pulse is a single pulse with little or noringing.

It has been found that broadband antenna structures accomplish this.FIG. 7 shows the antenna structure of one embodiment of the presentinvention. Both the transmit monopole 88 and receive monopole 90 are inseparate, rectangular cavities respectively 92, 94 in a metal enclosure98, which cavities provide isolation and semi-broad bandwidth.

Termination resistors R1, R2 and R3, (not the same as similarly labelledcomponents in FIG. 6) are selected to minimize any ringing. Thedimensions 11, 12, 13 are respectively 2.0", 1.5", and 1.0". The lengths14, 15 of respectively monopoles 88, 90 (see FIGS. 8 and 9) arerespectively 1.2" and 1.2".

FIG. 8 shows the transmitter pulse generator 40 and the transmittermonopole 88 and termination components R1, R2 and C3 of FIG. 7. Thepulse generator 40 includes transistor Q1 having a value of f_(T) (unitgain cutoff) greater than or equal to 7 GHz. The base of transistor Q1is connected as shown via a capacitor to the variable delay generator 38of FIG. 2.

FIG. 9 shows in addition to the receiver sampler 54 and the associatedpulse generator 44, which includes pulse generator transistor Q2 asdriven by the fixed delay circuit 42, the receive monopole 90 with itstermination transistor R3 as in FIG. 7. Transistor Q1 is similar totransistor Q2. The sampler 54 includes two Schottky diodes D1 and D2.One terminal of the hold capacitor 56 is connected to the amplifier 58of FIG. 2. It is to be understood that the values for the componentshown in FIGS. 8 and 9 are exemplary and not limiting.

Monopole antennas, as used herein, are linearly polarized, so that whenattempting to detect rod-like objects such as rebar, the axis of theantennas 88, 90 needs to be parallel with the length of the rebar formaximum detection. If the unit 32 were rotated 90 degrees such that theaxis of the antennas 88, 90 were perpendicular to the length of therebar, the signal return would be minimal. This property could be usefulin determining the rebar orientation within a medium (i.e. concrete),but requires the operator to be aware of this parameter and search inmultiple rotated axes.

If circularly polarized antennas as known in the art were utilized (notshown), both for the receiver and transmitter, the orientation of a rodtype target would not matter, as the circular polarization of the waveswould not differentiate between various rod rotation positions.

Another antenna embodiment (also not shown) uses circularly polarizedantennas which allow various target configurations to be at anyorientation without comprising the return echo strength.

If the target is a material surface (such as sheet metal) of a sizeequal to or larger than the antenna cavities' frontal area, antennacircular polarization (CP) yields no benefits and in fact produces aloss of 3 dB per antenna as CP type antenna structures are well known toproduce. Therefore, the choice of antenna for a given sensing situationshould take into account the physical configuration of the intendedtarget, and hence various antennas are applicable in accordance with theinvention.

A system and method in accordance with the present invention is notlimited to the particular applications described above, but has otherapplications for sensing of objects, not limited to the constructiontype field. For instance the system is applicable to sensing objectslocated in air that are relatively near the unit for purposes ofsecurity systems, traffic control situations (e.g. for installation inautomobile to detect proximity to another automobile or other object),and for other applications requiring detection of objects at arelatively short distance (within 200 feet) where a low cost system isimportant. This disclosure is illustrative and not limiting. Furthermodifications will be apparent to one of ordinary skill in the art, andare intended to fall within the scope of the appended claims.

I claim:
 1. A range swept radar system adapted for sensing a nearbytarget, comprising:a signal transmitter; a receiver located forreceiving the signals emitted by the transmitter after being reflectedfrom the target; a variable delay for sweeping a selected one of theemitted and received signals; means for stepping the variable delay inpredetermined increments; means for determining at each increment areceived amount of a clutter background signal not reflected from thetarget; means for storing the determined amount of the background signalfor each increment; and means for subsequently correcting a receivedsignal at a particulars variable delay by the stored amount.
 2. Atransmitter range swept radar system adapted for sensing a nearby targetcomprising:a signal transmitter driven by a continuously variable timedelay circuit; and a receiver driven by a fixed time delay circuit andlocated for receiving the signals transmitted by the transmitter afterbeing reflected from the nearby target; wherein a range gate time of thetransmitter is variable and a range gate time of the receiver is fixed.3. The system of claim 2, further comprising:means for stepping thevariable time delay in a variable delay for sweeping a selected one ofthe emitted and received signals; means for stepping the variable delayin predetermined increments; means for determining at each increment areceived amount of a random, reflected energy background signal which isclutter not from the target; means for storing the determined amount ofthe background signal for each increment; and means for subsequentlycorrecting a received signal at a particular variable delay by thestored amount.
 4. A range swept radar system adapted for sensing anearby target comprising:a signal transmitter driven by a continuouslyvariable delay circuit; a receiver located for receiving the signalstransmitted by the transmitter after being reflected from the nearbytarget; means for stepping the variable delay in predeterminedincrements; means for determining at each increment a received amount ofa random, reflected energy background signal which is clutter not fromthe target; means for storing the determined amount of the backgroundsignal for each increment; and means for subsequently correcting areceived signal at a particular variable delay by the stored amount.